The present invention relates to docklevellers, and in particular to a hold-down device used to lock the dockboard at a given elevated or horizontal or declined position.
Docklevellers are used to provide a ramp between the deck of a truck and the loading bay of a warehouse or the like, in order to facilitate loading or unloading. Docklevellers provide both a compensation for the height differential between the two levels and a bridge for the gap existing therebetween.
Various types of docklevellers are known. For the most part they include a base frame mounted in a recessed pit in the loading bay, a dockboard pivoted at its inner end to the base frame, and a dock-raising mechanism. In this manner, the dockboard is movable between a horizontal cross-traffic position, wherein the dockboard is flush with the loading bay, an elevated loading position wherein the dockboard extends to a vehicle platform higher than the loading bay, and a lowered, declined position wherein the dockboard extends to a vehicle platform which is lower than the loading bay.
In order to more effectively bridge the gap intermediate the dockboard and the vehicle bed, a lip extension is hinged to the front end of the dockboard. The lip extension is moveable between a vertical hanging position and a near horizontal position respectively as the dockboard is raised between its horizontal and elevated positions. For this purpose a lip extending mechanism is employed such as the one disclosed in U.S. Pat. No. 3,685,076. In this case the lip extending mechanism is actuated by the dock-raising mechanism so as to extend the lip only when the dockboard is raised higher than the level of the vehicle bed. The dockboard is then lowered to its loading position, at which point the now horizontal lip contacts the vehicle bed.
The dock-raising mechanisms known in the prior art may be manually or mechanically operated, or may utilize a hydraulic or pneumatic cylinder. They may operate directly between attachment sites on the dockboard and base frame, or between the base frame and a lever-link mechanism. A dockboard powered by a hydraulic cylinder operating between the base frame and a lever-link mechanism is more fully described in U.S. Pat. No. 3,694,840.
Today warehouses are often constructed by development companies wishing to avoid the additional expense of the power cylinders in the docklevellers. This change-over to the mechanically powered systems has presented problems in the industry. Whereas the use of the power cylinders provides a built-in hold-down device in the docklevellers, the use of the mechanical raising mechanisms requires a separate reliable hold-down.
In most mechanically operated docklevellers springs are used to provide a continuous lifting force to the dockboard. In order to lock the dockboard either at its horizontal cross-traffic position, at an elevated loading position, or at a declined loading position, the locking function of the hold-down device is required. The hold-down must additionally be releaseable to allow the dockboard to move between positions.
Presently the hold-down devices used in the spring-actuated docklevellers are mechanical, employing complex ratchet devices or friction bars to perform the locking and releasing functions. These mechanical hold-downs however are subject to slippage and other mechanical failures.